Method of resisting digester corrosion



y 1950 F. H. RICHTER 2,938,825

METHOD OF RESISTING DIGESTER CORROSION Filed Jan. so. 1953 ATTORNEY METHOD OF RESISTING DIGESTER CORROSION Frederick H. Richter, Glen Cove, N.Y., assignor to The Bahcock 8: Wilcox Company, New York, N.Y., a corporation of New Jersey Filed Jan. 30, 1953, Ser. No. 334,132

Claims. (Cl. 162-48) The present invention relates to the digestion of cellulosic materials in alkaline cooking liquors, and more particularly to methods for retarding the corrosion of the .inner face of a digester shell during one phase of the operating cycle. The invention is particularly concerned with methods of operation of a wood chip digester whereby localized concentration of cooking liquor by evaporation of water content during the liquor charging phase of the cycle is minimized.

In the pulp and paper industry wood chips are treated in digesters with a cooking liquor under controlled conditions of temperature, pressure, and time. The cooking liquor in the sulphate process of pulp production is predominantly composed of an aqueous solution of sodium hydroxide and sodium sulfide, and under digestion conditions dissolves the interfiber bonding materials of the wood whereby the fibrous material may be subsequently separated from the bonding materials which are in solution in the residual black liquor.

During recent years it has been observed that the corrosion rate of the inner face of the metal walls of the digester shells have been greatly accelerated in the sulphate digesting processes although when sulphides are absent, as in soda digesting process, the corrosion problem has not experienced a corresponding change. The increase in corrosion has reduced the life of the digesters and greatly increased that portion of the cost chargeable to this element of the plant in the production of pulp and paper. The reason for the accelerated rateof corrosion is unknown, but has occurred universally and has adversely affected the useful life of mild carbon steel, as well as alloyed steel, digester shells. Many attempts have been made to select a suitable alloy for installation in digesters to reduce or eliminate corrosion. Generally speaking, these attempts have not been successful, in that the increased life of the alloy metal has not justified the increased cost of the alloys.

I have found that one of the primary causes for the corrosion of digester shells is a direct or indirect result of the concentration of the liquor on the shell of a digester. Under operating procedures sufiicient heat is stored in the body of the digester to cause evaporation and concentration of cooking liquor contacting the digester shell in small quantities. Small quantities of liquor usually contact the hot walls of the digester as a result of splashing of the liquor stream during recharging of the digester and are concentrated by evaporation. The concentrated liquor in contact with the inner face of the shell apparently causes corrosive attack of the shell metal.

In accordance with my present invention, induce the rate of corrosion of the inner face of a digester shell by avoiding the evaporative concentration of the cooking liquor contacting the hot walls of the digester. This can be accomplished by changes in the digester charging procedure to avoid contact between small amounts of cooking liquor and the hot shell, by additions to the di- States Pate 7 2,938,825 Patented ,May- 31, .1950

2 gester to minimize the heat storage effect of the digester shell upon the cooking liquor, or by cooling the digester to reduce the temperature of the shell prior to theintroduction of the liquor so as to avoid surface or hot face bo'ding of the cooking liquor on the inner surface of the digester shell.

The various features of novelty which characterize my invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawings and descriptive matter in which I have illustrated and described an embodiment of my invention.

Of the drawings:

Fig. 1 is an elevation, in section, of a digester type commonly used in the pulp and paper industry; and

Fig. 2 is an elevation, in section, of the upper portion of the digester of Fig. 1 arranged for the introduction of wood chips and cooking liquor.

While =1 illustrate one common type of stationary pulp digester, it will be understood that other types of digesters, such as rotary digesters, are known and in use in this country and abroad. However, the present invention is applicable to all types of pulp digesters and for purposes of illustration the digester shown is typical f the operations involved.

As shown, the shell '10 of the digester is of circular cross-section having a domed top 11 and an inverted frusto-conical bottom portion 12. The dome 11 is provided with a central opening 13 for the admission of wood chips and cooking liquor. The opening 13 is provided with a cap member 14 which fits over the neck of the opening in pressure tight relationship therewith. In the embodiment shown, cooking temperatures and pressure within the digester are attained after charging by the introduction of steam through a pipe 15 and a steam distributor coil 16 which is disposed in the lower portion of the digester shell. The flow of steam to the digester is regulated by manipulation of the valves 17 and 18 in the pipe 15. The digester is provided with a pressure relief connection 20 located in the domed top 11, which is normally closed and is opened to reduce the pressure in the digester when the cook is completed. The bottom portion 12 is provided with an axially positioned outlet connected with a valved discharge pipe 21 for the discharge of the pulp and residual liquor from the digester at the end of the cooking period.

In recharging the digester, the cap member is removed and a hopper shaped Wood chip charging spout 22 is inserted into the opening 13. The spout 22 receives chips by gravity fiow from an overhead bin and is provided with a liquid nozzle 23 which is bent to project through the side of the spout and to connect with a controlled source of cooking liquor. In the customary procedure of operation the digester is progressively filled with cooking liquor at a temperature of approximately to F., While simultaneously the wood chips are also being introduced into the digester. In the sulphate or kraft process, white liquor consisting of sodium hydroxide and sodium sulphide is utilized as the cooking liquor although it is sometimes desirable to add a small amount of residual black liquor from a previous cooking cycle to the new cook. The digester body is filled with wood chips and liquor to a level spaced below the top 11, and when this level is reached, the spout 22 is removed and the digester is closed by the cap 14, and steam is supplied through the coil 16. 1

In some systems of digester operation the wood chips may be delivered to the digester before the liquor is introduced. Other units are operated with a small portion of the liquor charge being first introduced and thereafter the Wood chips and liquor are simultaneously introduced until the digester is filled.

. Thedelivery of steam to the filled digester displaces air which may be trapped in the submerged chips'and present in the pores of the wood chips, as Well .as that which is present within the shell of the digester above the liquor level. During the displacement of the air by steam certain vapors such as turpentine and acetone are liberated, with both the gas and the air discharged through the vent connection 20.

It is common practice to bring the pressure Within the digester up to 120 to 13 p.s.i. with a corresponding steam saturation temperature of from'340- to 350 F. This is accomplished in a period of from one to two hours, and the temperature of the shell is raised correspondingly with the inner surface being at the saturation temperature inasmuch as there is little heat loss from the outside surface of the shell. After the desired temperature and pressure conditions have been obtainedthe steam flow is reduced to a value suflicient to compensate for radiation losses, and the digester is maintained at the desired temperature and pressure condition for a cooking period of from 1 to ,3 hours.

.As illustrated in the drawing, the digester is arranged for the direct introduction of steam into the digester by means of the distributing coil 16. It will be understood that in some cases cooking pressure and temperature is obtained within the digest by the circulation of liquor from the digester through a heat exchanger to be heated therein by steam and returned to the digester.

After maintaining the digester at the selected temperature and pressure for the desired period of time the steam supply through pipe 15 is cut ofi and the pressure is usually reduced by exhausting steam through the vent connection 20, and thereafter the discharge pipe 21 is opened and the contents of the digester discharged for subsequent processing. This procedure ordinarily takes from to minutes and immediately thereafter the digester is available for recharging with liquor and chips, so that the batch cooking process can be repeated.

It is common knowledge that corrosion and erosion in pulping digester is not uniform over the inner surface of the digester shell. Extensive inner surface areas of a digester shell may corrode at a high rate while other areas in the same vessel may show an inconsequential corrosion rate or no corrosion at all. This condition also occurs generally regardless of the composition of metal used in constructing the digester shell. Generally, the shell areas showing no corrosion, or a low rate of corrosion, are found to be covered with a scale consisting primarily of calcium carbonate.

, It has not been possible to predict in advance of service Where the areas of rapid corrosion of a digester shell may occur. Sometimes the greatest corrosion in the shell will be in the so-called vapor zone i.e. above the level of liquor within the shell, and sometimes maximum corrosion occurs at elevations below the liquid level in the digester. No generally acceptable reason has been advanced for the accelerated corrosion in some areas of the digester shell and the substantial absence of corrosion in other areas of the same vessel. Some of the reasons suggested for localized corrosion may be due to vapors released during cooking, chip erosion during the digester charging period, temperature stresses or shock, residual welding stresses, type of wood cooked, differences in steel analysis of the plates utilized in forming the shell, and electrolytic action.

I have found the temperature in the'digester shell im-' It will be noted that ordinarily a digester will be from 40 to 50 feet long, and from 10 to 15 feet in diameter. With these dimensions and the customary operating pressure utilized in the digestion process the digester shell will be approximately 1 and A inch thick. This thickness of the shell provides an appreciable heat storage capacity, so that the shell 10 does not cool very rapidly by heat exchange .to the atmosphere unless some special provisions aremade.

in many digester installations the apparatus used for introducingthe cooking liquor into the digester shell permits small amounts of liquor to splash or otherwise come in direct contact with the inner surface of the shell. Under such circumstances the cooking-liquoris heated by heat previously stored in the digester shell metal by reason of its 340 F. temperature and the heating results in evaporation of water and an increase in the concentration of the chemical constituents of the liquor. Essentially, this isa condition .of surface or hot face boiling resulting from the heat stored in the shell 10 .of the digester.

I have found that boiling of'the liquor on the hot digester shell surfaces, .apparently with resulting liquor chemical concentration, accounts for localized corrosion in the vessel. 'Where boiling occurs, scale is absent and the corrosion rate is accelerated. Where boiling does not occur shell corrosion may be absent, or present in a rela tively small amount, depending upon the composition of the metal used in the digester shell.

When the liquor is introduced into the digester in such a manner as to result in the formation of a quiescent pool within the confines of the digester, the rising level of the liquor pool provides such a mass of liquor that the heatreceived from the shell with which it contacts is insufiicient to heat the mass to a degree to cause much if any evaporation and thereby any localized increase in concentration of the chemical constituents of the liquor.

A satisfactory procedure for absorbing heat from the inner face of the digester shell with a desired minimum of evaporation of the water of the liquor may be accom plished by introducing the liquor into the lower end of the digester and causing it to form a quiescent pool with a gradually increasing level until the digester has been filled. The cooking liquor introduction may be accomplished after the 'WOOd chips have been charged into the digester, as disclosed and claimed in my abandoned application, Serial No. 334,134, filed Jan. 30, 1953.

The temperature of the metal shell iii of the digester may be controlled in order to avoid surface boiling of the liquor by externally or internally contacting the digester shell with a cooling medium, such as for example water. External cooling of the shell can be accomplished by watersprays discharged from an annular manifold 24' positioned adjacent the upper portion of the shell. Internal cooling can .be accomplished by the insertion of a spray nozzle through theopening 13. When a water spray is used for cooling purposes the heat removed from the digester shell is lost insofar as the pulping process is concerned. While such heat loss is uneconomical, it

may be justified in some situations due to the improved life of the digesters. It is also possible to modify the temperature of thedigester shell by use of a cooling medium other than water, such as for example by weak residual black liquor. The 'black liquor can be sprayed or otherwise introduced into the digester to cool the shell 10 to a temperature of approximately 220 F., prior to the introduction of the cooking liquor used for the pulping process, withthe heated black liquor thereafter concentrated and incinerated for heat and chemical recovery. The "black liquor serves primarily as a heat absorbing fluid whose corrosive characteristics are not modified by concentration due to evaporation of the water content thereof. While heat will be recovered in the weak black liquor under these circumstances the capacity of the digesters will be reduced to the extent of the time required to cool the digester shell.

Since the primary cause for shell corrosion lies in the effect of small streams or a spray of cooking liquor contacting the hot metal wall of the digester, this adverse result can be avoided by the insertion of an inner liner within the digester shell. With the inner liner constructed and arranged as disclosed in my joint copending application with Frederick B. Snyder, Serial No. 334,133, filed January 30, 1953, the streams of cooking liquor do not directly contact the thick shell wall due to the use of a supplementary inner lining of lower heat storage capacity whereby surface boiling and corrosion of the shell wall is avoided.

It will be noted I have disclosed several methods for reducing or avoiding corrosion in pulp digesters used in the sulphate pulp cooking process. These methods all are characterized by the avoidance of contact between small bodies of cooking liquor with the inner surface of the relatively hot wall of the digester shell, which has a high heat storage capacity.

While in accordance with the provisions of the statutes I have illustrated and described herein the best form and mode of operation of the invention now known to me, those skilled in the art will understand that changes may be made in the process without departing from the spirit of the invention covered by my claims, and that certain features of the invention may sometimes be used to advantage without a corresponding use of other features.

What is claimed is:

1. In the cyclic batch cooking of cellulosic materials in a ferrous metal digester vessel the method of operation which comprises the steps of cooking a charge of said cellulosic materials and a cooking liquor at a predetermined temperature of at least 275 F. and superatmospheric pressure for a selected period of time, blowing the digester to remove substantially all of the charge, cooling the digester vessel to a temperature below the boiling temperature of the cooking liquor by contacting said vessel with a non-corrosive cooling liquid, and recharging the digester vessel with cellulosic materials and cooking liquor to repeat the process.

2. In the cyclic batch cooking of cellulosic materials in a ferrous metal digester vessel the method of operation which comprises the steps of cooking a charge of said cellulosic material and a cooking liquor containing an aqueous solution of sodium hydroxide and sodium sulphide at a predetermined temperature and a superatmospheric pressure for a selected period of time, blowing the digester to remove the charge, cooling the digester vessel to a temperature below the boiling temperature of the cooking liquor by contacting said vessel with a noncorrosive cooling liquid and recharging the digester vessel with cellulosic materials and cooking liquor after cooling said vessel to repeat the process.

3. In the cyclic batch cooking of cellulosic materials in a ferrous metal digester vessel the method of operation which comprises the steps of cooking a charge of said cellulosic materials and a cooking liquor containing an aqueous solution of sodium hydroxide and sodium sulphide at a predetermined temperature and at a superatmospheric pressure for a selected period of time, blowing the digester to remove the charge, cooling the digester vessel to a temperature of approximately 220 F. by a spray of cooling water contacting the external surface of said digester vessel, and recharging the digester vessel with cellulosic materials and cooking liquor to repeat the process.

4. In the cyclic batch cooking of cellulosic materials in a ferrous metal digester vessel the method of operation which comprises the steps of cooking a charge of said cellulosic materials and cooking liquor containing an aqueous solution of sodium hydroxide and sodium sulphide at a predetermined temperature at least 275 F. and a superatmospheric pressure for a selected period of time, blowing the digester to remove substantially all of the charge, cooling the digester vessel by heat exchange with residual black liquor whereby the vessel walls are cooled below the boiling temperature of the cooking liquor, removing the heated residual black liquor from said digester vessel, and thereafter recharging the digester vessel with cellulosic materials and cooking liquor to repeat the process.

5. In the cyclic batch cooking of cellulosic materials in a ferrous metal digester vessel the method of operation which comprises the steps of cooking a charge of said cellulosic materials and cooking liquor containing an aqueous solution of sodium hydroxide and sodium sulphide at a predetermined temperature of at least 275 F. and a superatmospheric pressure of a selected period of time, blowing the digester to atmospheric pressure and discharging the cooked charge of liquor and pulp, recharging the digester vessel with cellulosic material, extracting heat from the walls of the digester vessel by heat exchange contact with a cooling liquid to lower the temperature to the boiling point of the cooking liquor, and recharging the vessel with cooking liquor to repeat the process.

References Cited in the file of this patent UNITED STATES PATENTS 443,922 Smith Dec. 30, 1890 1,257,290 Lundberg Feb. 19, 1918 1,669,234 Dunbar May 8, 1928 1,761,543 Spencer June 3, 1930 1,859,384 Dunbar May 24, 1932 1,911,608 Davis et a1. May 30, 1933 1,987,214 Richter Ian. 8, 1935 2,389,246 Davey Nov. 20, 1945 2,410,964 De La Roza Nov. 12, 1946 2,474,862 Richter July 5, 1949 2,614,923 Tarkkonen Oct. 21, 1952 2,671,727 Westcott et al. Mar.-9, 1954 OTHER REFERENCES Pulp and Paper Manufacture, volume I, pages 320, 327, 328, published by McGraw Hill, New York, in 1950.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No ,v 2,938 825 Frederick H. Richter appears in the printed specification correction and that the said Letters May 31, 1960 It is hereby certified that error of the above numbered patent requiring Patent should read as corrected below.

Column 3 line 58, for "liquid" read liquor column second occurrence 6 line 2 strike out "at" Signed and sealed this 6th day of December 1960.

(SEAL) Attest:

KARL H. AXLINE ROBERT C. WATSUN Commissioner of Patents Attesting Officer 

1. IN THE CYCLIC BATCH COOKING OF CELLULOSIC MATERIALS IN A FERROUS METAL DIGESTER VESSEL THE METHOD OF OPERATION WHICH COMPRISES THE STEPS OF COOKING A CHARGE OF SAID CELLULOSIC MATERIALS AND A COOKING LIQUOR AT A PREDETERMINED TEMPERATURE OF AT LEAST 275 F. AND SUPERATMOSPHERIC PRESSURE FOR A SELECTED PERIOD OF TIME, BLOWING THE DIGESTER TO REMOVE SUBSTANTIALLY ALL OF THE CHARGE, COOLING THE DIGESTER VESSEL TO A TEMPERATURE BELOW THE BOILING TEMPERATURE OF THE COOKING LIQUOR BY CONTACTING SAID VESSEL WITH A NON-CORROSIVE COOLING LIQUID, AND RECHARGING THE DIGESTER VESSEL WITH CELLULOSIC MATERIALS AND COOKING LIQUOR TO REPEAT PROCESS. 